The present invention generally relates to an optical radiation sensor device, and more particularly to a sensor device incorporating a photosensor.
Light sensors are used in a large number of different applications. In such light sensing applications, several characteristics of the sensing mechanism need to be in acceptable ranges and some further need to be characterized for specific light sensing applications. Other characteristics of the sensor may increase the range of applications for which the sensor is suitable and/or may provide for easier or more economical design applications. One characteristic for which general requirements vary significantly from one application to another is the angular response characteristic, i.e., the angular response profile, of the sensor which is needed for the particular application. A second characteristic is the optical gain, which for low light level measurements is preferably high enough to make stable measurements of the lowest light levels which need to be detected by the system. A third characteristic is the need to provide a relatively small, aesthetically attractive, space efficient aperture in the device for entrance of the light to be measured. A fourth characteristic is to allow substantial and preferably variable distance to separate the aperture from the electronic sensing device. A fifth characteristic is to utilize separate components to sense the light and to characterize the angular response characteristic so that the sensor may be used in a broad range of applications leading to increased standardization of the light sensing component.
Sensor devices of the type used to detect light are constructed in a variety of packages. For example, photoresistive sensors are often mounted on a circuit board with or without a separate lens positioned in front of the sensor. Some photodiodes have been constructed in which the sensor die is mounted to a lead frame and is encapsulated by a clear epoxy. A portion of the epoxy encapsulant is molded into a lens so as to focus incident light onto the sensor die. Such lenses have been either spherical or other surfaces of revolution that are symmetric about an axis which is generally perpendicular to the surface of the active sensing element. Unlike a sensor construction in which a separate lens is spaced from the sensor, the lens in these types of sensor devices is an integral part of the sensor and the space separating the sensor and the lens has been eliminated. The main design difference which results from filling the space between the lens and the sensor with plastic is that the speed of propagation of the light rays is reduced being inversely proportional to the index of refraction of the lens material. This effectively increases the focal length of the lens in proportion to the index of refraction of the material.
FIGS. 4a and 4b illustrate two general sensing configurations, each with similar angular response characteristics but with widely differing optical gains. In the first sensor configuration in FIG. 4a, the sensor is close to the aperture and has desirably high optical gain. Placement of the sensor close to the aperture often leads to the added cost of additional parts and assembly processes, and longer electrical connecting paths to the sensor often compromises the electrical design. In the second sensor configuration in FIG. 4b, the sensor is placed at an appreciable distance from the aperture and has undesirably low optical gain. The placement of the sensor may be convenient and less costly but for the overall design the reduction in optical gain, which may be severe, may compromise or even prevent satisfactory performance.
The angles between lines 41a and 42a and between lines 41b and 42b are the same in each of the illustrative examples and denote the nominal angle between the 50 percent response points in the optical angular response profile for each of the sensors. Light blocking portions of the housing 44a and 45a are depicted in FIG. 4A in fragmentary view on opposing sides of the aperture which contains a lens 43a. With the sensing element 48a placed closer to the case than the point 49a of intersection of the lines 41a and 42a which depict the optical aperture, the lens, possibly combined with diffusion and/or de-focusing, may serve to decrease the viewing aperture from the angle between lines 46a and 47a to that between lines 41a and 42a as targeted by the design. The lens 43a serves to concentrate light impinging on the sensor thereby increasing its optical gain. Thus, the desired reduction in the overall field of view is accomplished while increasing the optical gain of the system. The general requirement for this to work with a single, thin lens in a non-light piped mode is for the sensor 48a to be located closer to the aperture than the apex 49a of the conic surface depicted by lines 46a and 47a in FIG. 4A. The conic surface may be non-circular and is used only as a temporary gage for illustrative or design purposes. With the lens and/or filter removed, the conic surface is aligned in the required viewing direction and inserted as far as possible into the aperture opening which is provided. (The regions which are generally closer to the apertures than the points 49a or 49b may be referred to as the near field regions of the respective aperture.)
Light blocking portions of the housing 44b and 45b are depicted in FIG. 4B in fragmentary view on opposing sides of the aperture which contains a diffusing lens and/or surface 43b. In this case, sensor 48b is farther from the aperture than the apex 49b. The property of point 49b is similar to that of 49a. An alternative way to describe it is as the point on the sensor side of the aperture which is the most distant point from the aperture from which the full field for which the sensor should respond to incident light or a substantial portion thereof may be seen prior to placing an optical element in the aperture. In this case, the sensor 48b is more distant from the aperture than the point 49b so that the angle between lines 46b and 47b is less than the angle between lines 41b and 42b. In three-dimensional terms, the solid angle subtended by the aperture at point 48b where the sensor is located is smaller than the solid angle subtended by the aperture at point 49b where the desired field for response to incident light may be seen through the aperture with the lens and/or filter removed. In this case, an optical element 43b, which has a diffusing effect, may be incorporated in the aperture and if the diffusing effect is pronounced enough to bend enough rays coming from representative directions 41b and 42b to the extent that they may strike the sensor 48b, a balance may be found for which the diffusing effect expands the effective viewing field from that indicated by the angle between 46b and 47b to that between 41b and 42b, as required to meet the design objective. The disadvantage is that instead of concentrating the light and adding optical gain as was accomplished in the first example, the light level is effectively attenuated because rays that would have come unobstructed through the aperture and struck the sensor before placing the diffuser in it are now spread out by the disbursing effect of the diffuser so that the proportion of the rays which reaches the sensor is diminished. Accordingly, there exists the need for a sensor device construction that may be placed within a housing a distance from an aperture through the housing without sacrificing optical gain.
One application for light sensors is as a glare sensor for a vehicle rearview assembly. Rearview assemblies may include an electrochromic mirror element and/or a display element. With an electrochromic mirror element, light levels sensed by a rearward facing glare sensor may be monitored and used to control the reflectivity of the electrochromic mirror element to prevent excessive glare from other vehicle headlights to be reflected to the eyes of the driver. The intensity of a display may likewise be varied as a function of the light levels sensed by a glare sensor.
U.S. patent application Ser. No. 10/068,540 discloses a sensor device that may be placed within a housing of a rearview assembly at a distance from an aperture through the housing without sacrificing optical gain. As disclosed in that patent, it may be advantageous to provide a diffuser across the aperture in the housing. In one particular embodiment, the sensor device is used as a glare sensor that senses light through an aperture formed in the bezel of a rearview mirror assembly where a diffuser may be provided in the aperture.
Although the construction disclosed in the '540 patent application works very well, some vehicle manufacturers prefer that the sensor device be positioned behind the mirror element or display element in order to minimize the apparent size of the bezel. In addition, some vehicle manufacturers are providing “theater seating” in some of their vehicles by which the rear seats are raised relative to the front seats. This has the consequence that it can reduce percentage of light from the rear window that may be sensed within the field of view of the glare sensor. Insofar as glare sensors sense an average light level across their field of view, theater seating reduces the average light level otherwise sensed by the glare sensor. In addition, in the same model vehicle, the seats may have either light or dark upholstery, which may affect the light levels sensed by the glare sensor when the seats are within the field of view of the sensor. Accordingly, there exists a need for a rearview construction by which the glare sensor may be positioned behind the mirror or display element and by which the performance of the glare sensor is improved to account for such positioning and to account for varying views from the rear window.